According to the damascene process, electrical circuits including metallic connections are placed within a layer of electrically insulating material present on a surface of a substrate. During a first step of the process, this layer is etched at a surface of this layer opposite the substrate, so as to produce trenches corresponding to the connections intended to be formed. During a second step, the layer of insulating material is covered with a coating of metallic material, such as copper, so as to fill the trenches. A polishing step is then carried out so as to remove a surplus of metallic material on top of the layer of insulating material. The process for fabricating the electrical circuit furthermore includes many other steps known to those skilled in the art, especially steps for producing semiconductor components such as transistors or diodes.
A complete electrical circuit produced on a surface of a substrate generally comprises several superposed layers of electrically insulating material, each of them incorporating metallic parts. Each layer is formed according to the preceding damascene process, which is carried out several times so as to obtain, in succession, all the layers, for example up to eight or nine superposed layers.
The “dual damascene” process constitutes a known variant of the damascene process. In this variant, the trenches etched within the layer of insulating material are distributed at two levels in the thickness of this layer. The trenches of the two levels are filled with metallic material during a single step of depositing this metallic material. The filled trenches belonging to the lower level—the one closest to the substrate—are generally intended to constitute electrical connections in a direction perpendicular to the surface of the substrate, usually called “vias”. The filled trenches belonging to the upper level—the one furthest from the substrate—are especially intended to constitute electrical connections in directions parallel to the surface of the substrate, usually called tracks. The present invention applies identically to the damascene process, as presented first, or to the dual damascene process.
Apart from obtaining metallic connections, the trenches produced in the electrically insulating material may be intended to form particular electrical components such as coils or inductors, especially RF coils or inductors, antennas, high-speed electrical signal transmission lines or capacitors having large plates. The contours of some of the etched trenches are therefore designed to correspond to these components. In the case of the dual damascene process, such components are generally placed in the upper level of the layer of insulating material.
For some applications, components, such as inductors, antennas, high-speed signal transmission lines or large capacitors, require that no other conducting part be present near each of these components. Indeed these components are sensitive to electrostatic coupling of the capacitive type and to inductive coupling, and their intrinsic operation, or the operation of the circuits which incorporate these components, is affected thereby. A volume of exclusion of conducting parts is therefore provided around these components, that is to say below and above these components and parallel to the surface of the substrate. Usually, the exclusion volume corresponds to a minimum distance of a few tens of microns from the contour of the component sensitive to electrostatic coupling.
The exclusion volume is occupied by the insulating material(s) used. These insulating materials may vary between two successive layers, but silica SiO2 is used more often than not, or else materials having a lower dielectric permittivity of the silicon oxycarbide (SiOC) type. Optionally, each layer of insulating material may contain parts made of a different insulating material such as, for example, silicon nitride (Si3N4) or silicon carbide (SiC). This is, for example, the case in particular in the dual damascene process, in which the lower and upper levels of trenches etched in one and the same layer of insulating material, for example silica, are separated by a silicon nitride barrier parallel to the surface of the substrate. This silicon nitride barrier separates the two superposed parts of the same layer of insulating material from each other, and does so outside the zones corresponding to trenches of the lower level. Such a silicon nitride barrier makes it possible to etch, in a manner known to those skilled in the art, during a single etching step, the trenches of both levels within the layer of electrically insulating material.
The exclusion volume surrounding some of the sensitive components causes a disproportion between the respective fractions of the insulating material and of the metallic material within each layer. This is because each layer is exclusively formed from insulating material within the exclusion volume, outside the sensitive component, whereas it includes a fraction of metallic material in the zones where other electrical components are distributed.
The polishing step in damascene and dual damascene processes uses a polishing liquid, called a “slurry”, and grit particles. The polishing liquid is introduced between that surface intended to be polished and a motor-driven disc having in general a plane surface and rotating in the plane of this surface. The grit particles, for example alumina grits, are either free in the polishing liquid or fixed to the surface of the motor-driven disc. The surface of the motor-driven disc is applied with a controlled pressing force against the surface of the substrate carrying the insulating material covered with metallic material. The objective of the polishing is to remove the surplus metallic material deposited on the insulating material, so as to expose parts of the insulating material between the trenches which remain filled with metallic material.
To reduce the polishing time, the polishing liquid includes chemical agents that are active with respect to the metallic material. These chemical agents modify the surface of the metallic material, forming a complex compound with certain components of the metallic material. This complex compound is then rapidly removed from the polished surface by the mechanical action of the polishing. The surplus metallic material may thus be rapidly removed, until the appearance of exposed portions of the surface of the insulating material. The insulating material is removed only slowly by the polishing, because of the absence of the complex compound formed by agents in the polishing liquid with components of the insulating material. Thus, the rate of removal of the metallic material is about 5 to 30 times higher than the rate of removal of the insulating material.
Because of this difference between the rates of removal of the metallic material and the insulating material, portions of insulating material that include different fractions of metallic material have different rates of material removal. Consequently, at the end of a polishing step, when the exposed surface includes zones of metallic material and zones of insulating material, more rapid removal of material takes place in the zones of metallic material, and therefore more material is removed therefrom, causing a surface planarity defect during polishing.
Such a loss of planarity of the surface of the insulating material occurs especially near the exclusion volumes associated with certain components. At the end of the polishing step, the upper surface of the insulating material in the exclusion volumes appears in relief with respect to the surrounding upper surface of the insulating material which incorporates parts of metallic material, creating a step or a change in level in this surface. This step or change in level may amount to 50 nanometres in height or more, and therefore causes parts of the surplus metallic material to remain, which parts cannot be removed during polishing because the grit particles cannot reach the internal angle of the step. Such parts of surplus metallic material that remain after polishing may extend as far as 200 μm from the step and cause short circuits and breakdowns during use of the electrical circuit.
U.S. Pat. No. 6,232,231 proposes to reduce, if not eliminate, the difference in material removal rate by placing metal inserts in those parts of the layer of insulating material not containing metallic circuits. These inserts or “dummies” have no function in the electrical circuit and are electrically insulated from the latter, especially from the functional metallic parts of this circuit. They are produced during the same steps of the damascene process as the metallic connections, namely during the step of etching the layer of insulating material, the step of filling with metallic material and the polishing step. These metallic inserts have the function of reducing the disproportion of the fraction of metallic material between various portions of the insulating material.
It is also known to combine such metal inserts with the dual damascene process (U.S. Pat. No. 6,214,745).
This first method of suppressing the risks of short circuits and breakdowns is incompatible with the exclusion volume surrounding components sensitive to electrostatic coupling, since the inserts themselves are metallic parts and consequently prohibited from the exclusion volume.
A second method of suppressing the risks of short circuits and breakdowns around the exclusion volume consists in providing an additional exclusion volume. Usually, such an additional exclusion volume extends the minimum distance separating certain metallic components from the component sensitive to electrostatic coupling up to 200 μm. This second method, although efficient, has the drawback of creating a large volume not used for the formation of components on top of the substrate, and this corresponds to an additional cost of the electrical circuit.